This invention relates generally to optical gain devices and, more specifically, to pump sources providing for unpolarized pump light for use in optical signal amplification in optical gain devices such as rare earth fiber amplifiers and Raman amplifiers or Raman distributed amplification in fibers using stimulated Raman scattering.
An optical gain medium is a device that, when provided with optical pumping energy, increases the amplitude of a desired optical signal propagating through such a device. Optical gain media may be constructed using optical fiber, such as that used for fiber lasers or fiber-based optical amplifiers or it may be a planar waveguide device having an embedded waveguide structure doped with a rare earth dopant. Forms of optical amplifiers known in the art are rare earth amplifiers, such as those formed from erbium or erbium doped fiber amplifiers (EDFA""s) and Raman amplifiers utilizing stimulated Raman scattering (SRS). In either case, optical pumping energy is injected into an optical fiber medium through which the signal to be amplified is propagating. The optical pumping energy, via either induced higher ion energy level or SRS, allows for a transfer of optical power to a wavelength longer than the pumping wavelength, due to the excitation of a vibrational mode in the medium that provides gain at the longer wavelength. A fiber laser is similar in functioning except that there is no signal to be amplified. In such a laser, erbium induced gain or Raman gain is developed between reflectors (which may be Bragg gratings in the case of glass fibers) provides a higher wavelength signal or a Stokes-shifted output signal of longer wavelength. A Raman laser or amplifier is a useful device for providing amplification at a target wavelength while pumped at a shorter wavelength. These types of amplifiers are particularly suitable for use in signal amplification in optical telecommunication systems.
It is desirable for pump sources for telecommunication applications to have the capability of power scaling and/or provide a depolarized output. In the case of an EDFA, multiple pump sources can be combined to provide an increase in pump power for purposes of enhanced signal amplification. In the case of Raman amplifiers multiple pump sources can provide both power scaling and also depolarized light output so that the signal to be amplified is capable of receiving continuous amplification energy in spite of changes in signal polarization as the signal propagates along the fiber medium.
While multiple pump sources, such as laser diode sources, are possible and have been combined in a convenient small package and employed as amplifier pump sources, they are not well suited as pump sources since laser diodes have polarized outputs and typically comprise a plurality of separate laser diodes which are bulky and are labor intensive to provide for combined laser diode light output.
A pump source constructed in accordance with the principles of the invention uses a dual stripe diode that generates two light beams with the same inherent polarization. A half wave plate rotates one beam so that it is polarized orthogonal to the other beam. Both beams are then applied to a polarization combiner that combines the beams to form an unpolarized beam that is focused to a single spot. The focused unpolarized output beam is utilized for pumping an optical amplifier, such as, for example, a planar waveguide amplifier, a rare earth fiber amplifier, a Raman fiber amplifier or for Raman distributed amplification.
In one embodiment, a birefringent crystal is used as the polarization combiner. The combined beams are then applied to a fiber via a lens, such as a graded index (GRIN) lens, used as a collimator. This arrangement can be integrated into a conventional housing, such as a xe2x80x9cbutterflyxe2x80x9d module or package, employing a cylindrical housing for the birefringent crystal and the GRIN lens fiber collimator.
In accordance with another embodiment, a dual stripe diode generates two beams with the same polarization. The beams are focussed on an optical device having two waveguides so that each beam enters one waveguide. A half wave plate introduced into one waveguide rotates one beam so that it is polarized orthogonal to the other beam. The waveguides are then fashioned into a conventional coupler or a Mach-Zehnder structure that combines the orthogonally polarized beams to provide an output comprising a single unpolarized beam.
In still another embodiment, a dual stripe diode generates two beams with the same polarization. Each beam is introduced into a polarization-maintaining (PM) fiber. One of the fibers is rotated 90xc2x0 so that, at the fiber beam output, the respective beam polarizations are orthogonal. The two polarization maintaining fibers are then fused to a fiber coupler, for example, a fiber Mach-Zehnder structure, that combines the orthogonally polarized beams to provide an output comprising a single unpolarized beam.
In accordance with still another embodiment, the power output of each polarization in the combined beam is measured and used to drive a feedback circuit that controls the current provided to each diode stripe in order to keep the power in the two polarization components substantially equal.